Chapter 1 What Is a Plant?
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OUP CORRECTED PROOF – FINAL, 02/17/12, SPi C h a p t e r 1 What is a plant? Plants, like love, are easier to recognize than to defi ne. At the entrance to many areas of outstanding natural beauty in England can be seen a sign that asks visitors to avoid ‘damaging trees and plants’. It is fair to ask in what way is a tree not a plant. A plant is often defi ned simply as a green, immobile organism that is able to feed itself (autotrophic) using photosynthesis. This is a heuristic defi nition for plants that can be refi ned if some more characters are added. Sometimes plants are described as organisms with the following combination of features: 1) the possession of chlorophyll and the ability to photosynthesize sugar from water and carbon dioxide; 2) a rigid cell wall made of cellulose; 3) storage of energy as carbohydrate and often as starch; 4) unlimited growth from an area of dividing and differentiating tissue known as a meristem; 5) cells with a relatively large vacuole fi lled with watery sap. So trees are clearly plants, and it is not diffi cult to think of other organisms that are unequivocally plants even though they lack one or more of these characteristics. For example, the orchid Corallorhiza wisteriana has the fl owers of an orchid, produces tiny seeds typical of the family Orchidaceae, and has the vascular tissue that you fi nd in the majority of land plants. However, what 1 OUP CORRECTED PROOF – FINAL, 02/17/12, SPi it does not have are green leaves, because this orchid is mycotrophic, meaning that it lives off fungi which themselves derive their energy from decaying material in the forest fl oor. It is able to do this because of a very intimate relationship with a fungus, a characteristic found to varying degrees throughout the orchid family. In a similar vein, Lathraea clandestina , which can be seen growing on the banks of the River Cherwell in Oxford, has fl owers reminiscent of a foxglove, yet it too has neither shoots nor leaves. Its fl owers emerge directly from the soil because this plant has roots that are able to infi ltrate the roots of willow trees and divert the nutritious contents of their vascular tissue. Both of these plant species have lost the ability to photosynthesize, but they are still plants because they share many, many other features with those plants which do still photosynthesize. The problem with the defi nitions above is that they are too limited, because they do not take into account some of the algae that live in water. In order to arrive at a sensible and unambiguous Plants defi nition for plants, we need to consider how we classify biological organisms. Similar individuals are grouped together into a species. Similar species are then grouped into a genus. Similar genera are grouped together into a family; and similar families are grouped into an order; similar orders into a class; similar classes into phylum; and similar phyla into a kingdom. Each of the groups in this hierarchy can be referred to as a taxon, and the study of groups is known as taxonomy. Prior to the 19th century, taxonomists tried to create a natural classifi cation that revealed the plan of the Creator. Since the 19th century, biologists have questioned whether species can change and evolve by retaining those changes and passing them on to their offspring. A great deal of work is currently being carried out to build the ‘tree of life’ (or phylogeny) that shows how all living organisms are related to each other. This work received its kickstart in 1859 with the publication of Darwin’s On the Origin of Species , and it is still ongoing. An evolutionary tree is the only illustration in 2 OUP CORRECTED PROOF – FINAL, 02/17/12, SPi What is a plant? 1 . Orobanche fl ava is one of many parasitic plants that do not photosynthesize but which steal from other plants 3 OUP CORRECTED PROOF – FINAL, 02/17/12, SPi The Origin , and Chapter 13 of the fi rst edition is still an eloquent introduction to taxonomy. Darwin talks about the possibility of building a natural classifi cation, but now natural means revealing the course of evolution and not the mind of God. Classifi cations now are based on what Darwin called commonality of descent . All the members of a taxon must share a common ancestor, and the group must contain all of the descendants of that ancestor. If these criteria are fulfi lled, then the group is said to be monophyletic. Monophyletic groups occur at every rank in the classifi cation from species to kingdom. If we see the evolution of species over the past 3,800 million years as a branching tree, then plants are one set of the branches on the tree of life, and this set of branches is all connected back to one crutch. The arguments start when you try to decide which crutch marks the start of plants. It is worth saying at this point that fungi are defi nitely not plants. Fungi are in fact on the branch next to animals on the tree of life. Despite this, mycologists (who study Plants fungi) do tend to be grouped with botanists rather than zoologists in university departments. The original plants At the heart of any defi nition of plants is the ability to photosynthesize. Unfortunately, there are organisms that photosynthesize but which cannot be considered by anyone to be plants. In particular, there are the photosynthetic cyanobacteria. It is currently believed that life has evolved just once and that this happened about 3,800 million years ago. At that time, the world as an environment for biology was very different. There was no protective ozone layer to absorb the harmful ultraviolet light from the Sun. Furthermore, the atmosphere contained a great deal of carbon dioxide but very little oxygen. 4 OUP CORRECTED PROOF – FINAL, 02/17/12, SPi The fi rst living organisms were simple compared to the majority of plants that we see around us today. For a start, they were unicellular. They were prokaryotes. There are many prokaryotic organisms still extant in two big groups: the archaea and the bacteria. (The other major group of organisms are the eukaryotes, that is, plants, animals, fungi.) Fossilized prokaryotes have been found in rocks dated at nearly 3,500 million years old. The fossils of these early bacteria are grouped in structures that look the same as the stromatolites that can be seen in several places around the world today. A stromatolite is a cushion-shaped rock that is found on the edges of warm shallow lakes, most commonly salt-water lakes, and they are (very simply) laminated accumulations of microbes. The colonies of the unicellular cyanobacteria live in a fi lm of mucus. Calcium carbonate builds up on the mucus and the cyanobacteria What is a plant? migrate to the surface and a new layer of mucus is formed. These alternating layers are then fossilized and the bacteria enclosed in the rocks. So it was clear to see that prokaryotic life had evolved perhaps as early as 3,800 million years ago, but it was not so easy to determine how these early living entities found the energy to live. Some may have synthesized enzymes to break down minerals, but this was slow. There is now compelling evidence that the cyanobacteria in these fossil stromatolites were able to capture the energy of the Sun and use it to synthesize molecules containing carbon derived from the abundant carbon dioxide in the atmosphere. This evidence is based around the fact that the enzyme that drives the capture of carbon from carbon dioxide preferentially fi xes one carbon isotope ( 12 C) over the other that is also present in the atmosphere ( 13 C). So if carbon compounds contain the two isotopes in different proportions from those in the atmosphere, then the compounds were the product of photosynthesis. Carbon compounds have been found in rocks in Greenland that have the carbon isotope ratio produced by photosynthesis. 5 OUP CORRECTED PROOF – FINAL, 02/17/12, SPi Photosynthetic organisms, with which we are familiar, use water as a source of electrons. The oxygen in the water is then released into the atmosphere as gas. It is thought that the fi rst photosynthetic cyanobacteria may have used hydrogen sulphide (H2 S) rather than water (H2 O). It is currently believed that by 2,200 million years ago, cyanobacteria were generating large amounts of oxygen and that this was accumulating in the atmosphere. This may seem like a small point, but the fact that cyanobacteria began using water as a supply of electrons led eventually to the levels of oxygen in the atmosphere that made aerobic respiration possible and the majority of biology as we know it. The generation of oxygen had another effect, namely the formation of the layer of ozone in the upper atmosphere, whose absence has already been noted and whose protective function is so important for biology. Prior to this, the mucus in the stromatolites may have helped to protect the cyanobacteria. Living in water would also have afforded some protection. Plants So to recap, we see that by 2,000 million years ago, there was a large population of prokaryotic cyanobacteria that was generating oxygen by photosynthesis, but there was still nothing that we could describe as a plant. The evolution of plants required an event that must have happened but for which we do not have a complete cast list.